The present invention relates to the field of implantable medical devices, including implantable electrodes. More particularly, the invention relates to cranial sealing plugs used to seal a hole made through the skull of a patient for the purpose of inserting a medical device and/or electrode into the brain. Even more particularly, the invention relates to burr hole plugs which are flush with the surface of the skull. The embodiments described in the present invention may be used with a wide variety of burr hole sizes, which burr holes are typically used to provide electrode or lead access to the brain in connection with a Deep Brain Stimulation (DBS) system. It is to be noted, however, that the burr hole plug embodiments of the invention may also be used to seal holes made for the passage of other objects into the brain, such as ablation catheters, drug delivery catheters, tissue delivery catheters, and the like.
Deep Brain Stimulation is a medical procedure which involves access to the brain through a burr hole placed in the skull for treating Parkinson's Disease, Essential Tremor, dystonia, chronic pain relief, and potentially the treatment of choice for other disorders. A typical surgical approach may include the following steps, of which only steps 8, 31, and 32 relate directly to the present invention:                1. Attach the stereotactic frame on patient's head.        2. Obtaining an image of the various brain structures with MRI or equivalent imaging modality of patient with stereotactic frame.        3. Obtaining target identification using a planning software.        4. Positioning the patient with stereotactic frame in head rest.        5. Cutting the skin flap, exposing the working surface area of cranium using scalp clips.        6. Placing the stereotactic arc with target coordinate settings and identifying the location on the skull for creation of the burr hole.        7. Removing the arc and drilling a burr hole.        8. Placing the base of the burr hole plug.        9. Placing the stereotactic arc with microelectrode recording drive attached and appropriate stereotactic frame adaptor inserted into instrument guide.        10. Inserting the recording microelectrode cannula and insertion rod approximately 25 mm above the target.        11. Removing the recording microelectrode insertion rod, while leaving recording microelectrode cannula in place.        12. Inserting the recording microelectrode such that the tip is flush with the tip of the recording microelectrode cannula.        13. Attaching the connector pin of the recording microelectrode to the microelectrode recording system.        14. Starting approximately 25 mm above target, begin the microelectrode recording tract using the microdrive to advance the microelectrode at a specified rate.        15. If the target is identified, prepare to place the stimulating macroelectrode and proceed with step 16, if the target is not identified proceed with the following:                    a. Using recording results and pre-operative imaging, determining a new set of coordinates for the theoretical target.            b. Disconnecting recording microelectrode from the microelectrode recording system.            c. Removing the recording microelectrode cannula and recording microelectrode.            d. Continuing with the surgical process at step 10 above.                        16. Removing the recording microelectrode cannula and recording microelectrode.        17. Inserting a larger diameter insertion cannula and rod (the tip of the slit insertion cannula and rod will be at a location 25 mm above target).        18. Removing the insertion rod, while leaving insertion cannula in place.        19. Inserting a stimulating macroelectrode, and advancing to the target stimulation site identified in step 15 above.        20. Using macrostimulation, simulating the stimulation of the chronic        
DBS lead to ensure proper results.                21. Removing the stimulating macroelectrode and cannula.        22. Inserting an insertion cannula and rod that accommodates the DBS lead.        23. Removing the insertion rod.        24. Inserting the lead with stylet through the insertion cannula.        25. Advancing the lead with the stylet to the target stimulation site.        26. Once it is determined that the lead has been advanced to the desired target location, connecting the connector of the lead to a trial stimulator.        27. Performing the desired stimulation and measurements using any one or combination of the four electrodes.        28. If results are favorable, then begin removal of the insertion cannula and the stylet, and proceed with step 29, if the results are not favorable then proceed with the following:                    a. Using the macrostimulation results, microelectrode recording results, and pre-operative imaging, determine a new set of coordinates for the theoretical target.            b. Removing the lead and stylet.            c. Removing the insertion cannula.            d. Adjusting the coordinates of the stereotactic frame.            e. Continuing with the surgical process at step 10 above.                        29. Removing the stylet followed by the insertion cannula.        30. Using macrostimulation verifying that microdislodgement of the lead has not occurred.        31. Locking the lead in the burr hole plug.        32. Placing the burr hole plug cap.        
Several inventions are known in the art that relate to burr hole plugs used for DBS application. As used herein, the term “burr hole plugs” relates to any device or assembly or system that includes one or more components for insertion into a burr hole. Typically, a burr hole plug assembly includes at least two components: a ring or grommet-like element that is inserted into the hole first so as to protect the edges of the burr hole (item 8 above), and a cap or plug device that is inserted into the ring or grommet-like element after the lead has been positioned in order to secure the lead and plug the hole.
By way of example, in U.S. Pat. No. 6,044,304, entitled Burr Ring with Integral Lead/Catheter Fixation Device, a burr ring that is secured to the skull portion of the brain is described. Patent '304 is incorporated herein by reference. The burr hole plug described has an aperture capable of accepting a lead through a septum. In addition the burr hole plug has an upper flange portion and circumferential ribs used to position the plug in the cranium. The flange feature prevents the burr hole plug from being flush with the surface of the skull. Use of the ribs in the manner suggested indicates that the material from which the plug and ribs are made is relatively rigid.
Another burr hole plug known in the art is described in U.S. Pat. No. 6,006,124, which patent is incorporated herein by reference. The '124 patent describes electrically connecting an implantable control module to a brain electrode. The '124 patent also describes a method of placing brain electrodes beneath the scalp. No mention is made in the '124 patent of a mechanical connection for the lead, nor of the mechanical stability of the burr hole plug.
Still, with reference to U.S. Pat. No. 5,954,687, which patent is incorporated herein by reference, an apparatus is disclosed for securing a catheter within a burr hole. The apparatus described in the '687 patent has a series of spaced septum elements that can be selectively penetrated for fluid communication with a reservoir in the apparatus. The main objective of the apparatus is to allow fluid access to the patients' brain through a burr hole. Anchoring of the device is not taught, and there are a limited predetermined number of septum holes that can be accessed, thereby limiting use of the apparatus.
In yet another example, U.S. Pat. No. 5,927,277, which patent is also incorporated herein by reference, describes a burr hole ring for retaining a probe relative to the skull. The burr hole ring has an engaging member with holes to receive a probe. The '277 patent also describes a method for securing a device at a desired orientation within the burr hole. Since a fixed spacing between holes is described, this limits the number of locations that the device can be placed through the burr hole. The device described in the '277 patent consists of several parts one of which contains a flange.
U.S. Pat. No. 5,865,842, incorporated herein by reference, teaches a system and method for anchoring a lead in a burr hole. Such system consists of a base-plate, adaptor, seal, and screw cap. The lead is anchored mechanically at the burr hole at a 90 degree angle relative to the burr hole. A flange protrudes from the burr hole of the skull.
In U.S. Pat. No. 5,843,150, incorporated herein by reference, an annular clamping means with a compressible feedthrough member for receiving a lead is described. The described order for anchoring the lead includes making the burr hole, inserting the plug ring, inserting the lead, and engaging the clamping member. The embodiments described show an annular clamping means with protruding flanges.
Other burr hole plug assemblies, and features of burr hole plugs are taught, e.g., in U.S. Pat. No. 5,464,446 (burr hole plug with a central lumen and a cap that engages with the flange of the plug); U.S. Pat. No. 4,998,938 (a device that facilitates insertion of an instrument into a patient's cranial cavity); U.S. Pat. No. 4,328,813 (a burr hole plug with a cap that anchors the lead); and U.S. Pat. No. 4,245,645 (a probe and system that is used to perform stereoelectroencephalographic exploration). The burr hole plug applications described in these additional patents, including many lead insertion techniques, may also be used with the present invention. For that reason, the '466, 938, '813 and '645 patents are incorporated herein by reference.
Despite the various types of burr hole plugs known in the art, as illustrated by the examples cited above, significant improvements are still possible and desirable, particularly relative to a burr hole plug assembly that: (a) can accommodate various burr hole sizes and a secure fit in the burr hole; (b) can accommodate various locations for lead positioning and adjustment; (c) can allow the lead to remain in a static position when the burr hole plug is placed; (d) protect the lead from fracture at the exit location of the plug; (e) is flush with the skull to avoid skin erosion and to eliminate any unsightly flange protrusion from the patients' skull (which can be especially important in the population of older balding males); (f) adequately holds the lead in place over time, eliminating the need for additional surgeries; and (g) provides a selection of various types of burr hole plugs that may be used by the medical industry, thereby eliminating the need for physicians to resort to custom devices.
It is thus seen that there is a continual need for a variety of burr hole plug assemblies and lead securing methods for use therewith.